[llvm-dev] A libc in LLVM

Thu Jun 27 16:57:10 PDT 2019

On 6/27/19 5:43 PM, Siva Chandra via llvm-dev wrote:
> On Thu, Jun 27, 2019 at 2:05 PM Chris Lattner via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Saleem, Owen, others on the thread who are concerned about this: it seems that some of the concern is that the project goals are too narrow, and thus the eventual result may not serve the full community well over time.
> May be my email listing our goals is being misinterpreted as being the
> bounding set of goals for the project. So, let me make it clear again:
> The goals I have listed are just our initial set of goals for the
> project. Members of the community are of course free to add their own
> goals to this set, implement them, and make it a "full solution." I
> have also mentioned in some of my earlier emails that we do not intend
> to design out any particular feature or platform. For example, I have
> said that we do not intend to work on dynamic linking/loading at least
> to begin with. This does not mean that the scope of the project is
> curtailed to static linking. The members of the community are free to
> add support for dynamic linking/loading. In fact, if dynamic
> linking/loading support is added in a modular/"as a library" fashion,
> it makes it a win-win situation as we will be able to take it out if
> we do not require it.
>

I think that it is important that we not, as a community, exclude from 
the project any libc implementation just because it does not aim to be a 
glibc or Windows CRT replacement. If people want that, then that's 
great, but there is significant value regardless.

One of my primary use cases for an LLVM libc is to take a subset of it 
and link it with our OpenMP device-side runtime library, or into code 
being compiled for CUDA/HIP/SYCL/etc. (so that we can support compiling 
code for accelerators (e.g., GPUs) that happens to call snprintf (or 
whatever) across platform from a variety of vendors). I believe that I 
can get this capability with only minor additional effort, so long as 
the libc is sufficiently modular. Being part of the LLVM project will 
make it much easier to ensure that this configuration is tested and 
supported.

  -Hal

>
>
>
>> -Chris
>>
>> On Jun 27, 2019, at 1:19 PM, Owen Anderson via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>>
>>
>> On Jun 27, 2019, at 2:53 PM, Saleem Abdulrasool via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>>> So, what do you think about incorporating this new libc under the LLVM project?
>>
>> As stated, I really feel that this is far too specialised to certain use cases that are pertinent to Google.  I think that this needs to be broadened to allow a general purpose libc much as libc++ is a general C++ implementation.  I think that the project has a different set of requirements and seems like it would be extremely interesting to see how it would develop over time.  This could really be an interesting choice for a certain type of project but as described feels like it is best explored outside of the umbrella of LLVM.
>>
>>
>> I don't have a strong stake in this decision, but Saleem's commentary matches my thoughts on the topic.  Maybe some of this is related to messaging - would the proposed project be *an* LLVM libc or *the* LLVM libc.  There is already at least one instance within the LLVM umbrella where a subproject designed and built to a particular set of constraints became *the* LLVM solution, and ended up disincentivizing investment from contributors whose priorities didn't match those constraints.  Staking the blessed-by-LLVM slot for a piece of the toolchain is not free.
>>
>> To turn the question around, why should *this* libc (assuming it will be built whether or not LLVM accepts it) be *the* LLVM libc?
>>
>> --Owen
>>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory